WASHINGTON — Once assured of victory on friendly turf, Mitt Romney faces a potentially devastating loss of his home state of Michigan to Rick Santorum, which could upend the campaign for the Republican presidential nomination.

Romney, the son of a popular former Michigan governor and a onetime Detroit auto executive, finds himself trailing Santorum in one of two states that vote Feb. 28. The other is Arizona, but Santorum’s concentrating most on Michigan. A Romney loss in Michigan could weaken him going into the 10 “Super Tuesday” states that vote on March 6 and throw the race wide open.

In response, Romney and his allies are unleashing an advertising blitz against Santorum in Michigan much like the ones they used in Iowa and Florida to defeat Newt Gingrich when he was the main threat to Romney’s campaign.

It could work again. Santorum is largely unknown. The former senator from Pennsylvania is about to be defined by hostile ads portraying him as a big-government Washington insider with no executive experience. And despite some recent success raising money, he still lacks the kind of cash he’d need to buy enough ads in Michigan to match Romney’s barrage.

Romney, the former governor of Massachusetts, has long counted on Michigan. He grew up there. His father, the late George Romney, chaired American Motors and served as governor from 1963 to 1969. Mitt met his wife, Ann, there; they still keep a summer home in the state. “Michigan’s been my home, and this is personal,” Romney says in one ad.

He won the Michigan primary in 2008 against John McCain, and for months polls suggested an easy repeat this time around. As recently as Feb. 2, he led by 15 points.

Yet since Santorum won three states on Feb. 7, he’s surged into the lead in Michigan, just as he’s pulled ahead in national polls.

The swing in fortunes is driven by a one-two punch. First, Romney’s failing to connect with some blocs of Michigan voters such as evangelicals and tea party supporters. Second, Santorum’s Feb. 7 wins gave him a burst of publicity just as many Michigan voters started tuning in and weighing an alternative to Romney.

“Santorum is ahead because he is the latest ABR — ‘anyone but Romney’ — candidate to emerge,” said Steve Mitchell, a Republican pollster in Michigan whose latest survey found Santorum leading Romney by 34 percent to 25 percent among likely primary voters. Gingrich and Rep. Ron Paul of Texas trailed far behind.

Santorum hopes to win Michigan with an appeal to the state’s blue-collar workers, emphasizing his personal tale of a grandfather who worked the coal mines of Pennsylvania and an economic agenda keyed to helping restore manufacturing jobs. He’ll speak Thursday at the Detroit Economic Club.

Santorum also hopes his social conservatism will help in a state where one of three primary voters could be fellow Roman Catholics.

“If Rick Santorum can beat Mitt Romney in Romney’s home state, it will really upset the apple cart,” said Whit Ayres, a Republican pollster.

A loss in Michigan would reinforce the idea that Romney cannot rally his party’s conservative base. And it could drive the Republican establishment — worried that Santorum’s very conservative stands on social issues would render him a weak general election candidate — to try to pressure a late-entry candidate into the race, such as former Florida Gov. Jeb Bush or Indiana Gov. Mitch Daniels.

“The panic among Republican elected officials we saw after Newt’s win in South Carolina would come back in full force,” Ayres said.

Romney and an independent group supporting him are launching a campaign to make sure that doesn’t happen.

“You have to lay out the differences between yourself and the people you’re running against,” Romney said Wednesday on Fox News.

Romney and his campaign are slamming Santorum as a former senator with no executive experience — a la Barack Obama — and one who voted for big-government spending, such as the infamous “Bridge to Nowhere” in Alaska.

“He’s never run anything,” Romney said Wednesday of Santorum. “I think we’ve seen what happens in a country when you have a president who has no experience running anything. That’s what we have now.”

At the same time, the political action committee that supports Romney, Restore our Future, has bought $640,000 worth of air time on Michigan TV stations for ads likely to blast Santorum’s record in Congress. The same group buried Gingrich in Iowa and Florida, both states where the former speaker of the House of Representatives had taken a lead in polls, only to fall under the cascade of negative ads.